1. Field of the Invention
The present invention relates to structural assemblies and, more particularly, relates to a method and preform for forming a complex contour structural assembly.
2. Description of Related Art
Honeycomb panels are used in a variety of applications, especially where strength and minimum weight are required. A conventional honeycomb panel typically includes a core between first and second skins or face sheets. The core is a cellular member, i.e., the core defines a number of internal cells. In some cases, each cell is six-sided and extends in a longitudinal direction parallel to the other cells of the core. However, other cores can also be formed and used. Such panels can be formed of a variety of materials, including aluminum, titanium, and the like.
Metallic honeycomb panels are typically formed in a flat or gently contoured configuration. For example, in some cases, the core is formed from a flat sheet and bonded to the face sheets during a combined forming and bonding operation. In particular, the core and face sheets can be superplastically formed and diffusion bonded or brazed to form a flat or gently contoured honeycomb panel. Such forming and bonding operations are described in U.S. Pat. No. 4,117,970, entitled “Method for Fabrication of Honeycomb Structures,” which issued on Oct. 3, 1978; U.S. Pat. No. 5,420,400, entitled “Combined Inductive Heating Cycle for Sequential Forming the Brazing,” which issued on May 30, 1995; U.S. Pat. No. 5,700,995, entitled “Superplastically Formed Part,” which issued on Dec. 23, 1997; U.S. Pat. No. 5,705,794, entitled “Combined Heating Cycles to Improve Efficiency in Inductive Heating Operations,” which issued on Jan. 6, 1998; U.S. Pat. No. 5,914,064, entitled “Combined Cycle for Forming and Annealing” which issued on Jun. 22, 1999; and U.S. Pat. No. 6,337,471, entitled “Combined Superplastic Forming and Adhesive Bonding” which issued on Jan. 8, 2002.
In some cases, the honeycomb panel can then be formed to a contoured shape, i.e., so that each of the face sheets define a two- or three-dimensional curve. The degree of contour that can be imparted to such a structural panel is dependent on such factors as the size and configuration of the panel, the material properties of the panel, the type of forming process, and the like. If excessively formed, undesirable changes in the configuration of the panel may result, such as crushing of the core, buckling of the face sheets, or destruction of the joints between the core and face sheets.
The structural assemblies for some applications define complex contours that are difficult or impossible to form by conventional manufacturing methods. For example, U.S. Pat. No. 6,371,411, issued Apr. 16, 2002, U.S. Pat. No. 6,457,676, issued Oct. 1, 2002, and U.S. Pat. No. 6,688,558, issued Feb. 10, 2004, each titled “Method and apparatus for aircraft inlet ice protection,” describe an ice protection system for an inlet for an aircraft engine, which inlet can be partially lined with a honeycomb core sandwiched between sheets, thereby improving the acoustic attenuation of the inlet. In some cases, the core is provided in multiple portions, in part, to facilitate manufacture.
Thus, there exists a need for an improved method and preform for producing metallic complex contour structural assemblies, including contours not typically formable by conventional methods. The method should be capable of producing structural assemblies with cellular cores and should be compatible with superplastically formable materials such as titanium.